Well gravel packing is a conventional oil field technique for screening formation sand and sediments from entering the well with the produced fluids which would clog up the well and detour or prevent production of the fluids. Basically, gravel packing refers to the petroleum industry practice of surrounding with gravel or coarse sand, the perforated liner through the producing formation in an oil or gas well. When properly placed in the annular space between the wall of the well and the perforated liner, gravel supports the walls, prevents caving of loose material against the liner and and serves to restrain sand from unconsolidated and disintegrating strata so that it may not enter the well. More effective screening of sand, possible by this means, diminishes the destructive influence of sand scouring on well equipment and tends to reduce maintenance costs. Equipment repairs and well clean-out operations are less frequent and the well is able to produce for a greater part of the time than would otherwise be the case. The well so protected is therefore capable of maintaining a larger average monthly production rate. With gravel to sustain the walls of the well, it is possible to form and maintain a hole of larger diameter through the producing formation without elsewhere increasing the normal diameter of the well or that of the well casing. The larger diameter hole through the producing zone results in increased production efficiency.
Although the gravel may be simply "dumped" into the well to fall into place to form the aforementioned annular column or gravel sheath, it is more commonly emplaced in the well by circulating the gravel in a fluid carrier which is pumped from the surface. The fluid returned from the well is recirculated by conventional drilling pumps through well fluid treating equipment and thence through a gravel pack machine which blends the prepared gravel in the circulating well fluid. The gravel carrying fluid is then pumped down into the well through the drill pipe to a cross-over tool wherein it is directed to the annulus between the perforated pipe or liner and the under-reamed hole below the cemented casing. The gravel-well fluid mixture then continues down the outer annulus of the perforated liner to the bottom of the hole or to the top of the gravel sheath which has accumulated at the bottom of the hole outside of the perforated liner as the packing progresses. Since the perforated wall of the liner offers less resistance to the fluid as it reaches the top of the gravel sheath, than does the gravel sheath, the fluid enters the inner annulus of the liner through the perforations therein and flows down through the annulus to the bottom of the fluid return tailpipe which is positioned concentrically within the perforated liner. The gravel, of course, is too large to enter the liner perforations or slots and thus remains in the outer annulus while the fluid passes up through the tailpipe to the cross-over tool in which it is directed to the annulus between the drill pipe and the casing, through which annulus it returns to the surface. Sufficient gravel is pumped into the under-reamed well to accumulate gravel in the well to completely envelop the perforated liner in a sheath of gravel several inches thick.
As the gravel enters the well and accumulates in the under-reamed hole, there is a tendency of the liner perforations to plug and for the gravel to bridge or dune before reaching the bottom of the free space, thus leaving voids in the gravel, which reduce the efficiency of the gravel pack in screening the formation sand and sediments from the well fluid entering the perforated liner. In order to cause the fluid carrying the gravel to move down through the outer annulus to the top of the gravel sheath before entering the liner, it is desirable to seal or blank off the perforations of the liner down to the top of the gravel sheath and then remove the perforation seal at that area so that the fluid will enter the liner only in this area. The tendency for gravel to bridge is particularly troublesome in slant drilling operations, however, maintenance of the fluid velocity down to the top of the gravel sheath reduces the tendency for the gravel to bridge and create voids in the gravel pack, and also aids in the compaction of the gravel.
Prior art efforts to limit entry of the well fluid to the top of the gravel sheath have included such techniques as plugging the perforations with litharge or sulfur, lining the liners with cement, and placing swab cups at every joint of the liner. Recent efforts have utilized thin rubber flanges at every joint, or a collapsible valve positioned at every six foot interval. None of these prior art attempts to limit the entry of the well fluid into the perforated liner at the top of the gravel sheath, has been completely satisfactory as some of the well fluid still enters the liner intermittently throughout its length. Also these prior art techniques reduce the speed of the gravel packing operation and increase the difficulty of the operations.
Accordingly, it is an object of my present invention to provide a gravel packing method wherein all of the liner perforations are temporarily plugged but opened at the top of the gravel as the level of the gravel rises in the well, thus permitting passage of well fluids into the liner at the upper level of the gravel pack sheath accumulating in the well.
It is also an object of my present invention to provide a liner perforation seal for use in gravel packing operations which prevents entry of gravel-carrying fluid into the liner.
It is also an object of my present invention to provide in a gravel packing operation, a means for temporarily sealing a perforated liner against the passage of well fluids into the liner and which will permit entry of the fluid at the vicinity of the top of the gravel sheath.
It is a further object of my present invention to provide a gravel packing method wherein the annulus between the perforated liner and the fluid return tailpipe is filled with a semi-solid material which is impervious to well fluid at normal pressures but pervious to well fluid at increased pressures.
It is also an object of my present invention to provide a ported tailpipe for gravel packing operations in which longitudinally spaced ports are sequentially opened at the level of the top of the gravel pack sheath to permit entry of the well fluids into the tailpipe at that level.